1. Field of the Invention
This invention relates, in general to magnetic core structures for electrical inductive apparatus, such as transformers, and more specifically, to magnetic core structures of the stacked type.
2. Description of the Prior Art
One example of a prior art magnetic core structure of the stacked type is disclosed in U.S. Pat. No. 3,153,215. That patent discloses magnetic core structures of the stacked type which have step lap joints between the mitered ends of the limb or leg and yoke portions of the magnetic core. In a step lap joint, the joints between the mitered or diagonally cut ends of the leg and yoke laminations, in each layer of the lamination, are incrementally offset from similarly located joints in adjacent layers in a predetermined step or progressive pattern, with the joints being stepped at least three times in one direction before the direction is changed or the pattern repeated. Magnetic cores with step lap joints have been found to substantially improve the performance of the magnetic core, compared to magnetic cores which utilize conventional butt-lap type joints by lowering the core losses, lowering the exciting volt-ampere requirements, and lowering the sound level of the magnetic core. Other prior art step lap joint arrangements are shown in U.S. Pat. Nos. 3,153,215; 3,477,053; 3,504,318 and 3,540,120. These patents disclose joint arrangements where the desired stepped relationship is obtained between diagonally cut ends of the laminations by providing laminations for each leg or yoke portion which have the same longitudinal dimensions between the diagonally cut ends. The stepped relationship is achieved by incrementally offsetting the mid-points of the laminations of any stacked group of laminations.
In prior art magnetic cores having stepped-lap joints, the stepped-lap joint between the inner leg and the top and bottom yoke laminations is constructed by forming a V-shaped notch in each of the top and bottom yoke laminations. The V-shaped notch in the yoke laminations is incrementally shifted, from layer to layer, parallel to the longitudinal axis of the magnetic core such that the inner leg laminations, which are of equal length, are also incrementally shifted parallel to the longitudinal axis or length of the magnetic core. In this manner, the equal length laminations of the top and bottom yokes are horizontally shifted from layer to layer which uniformly distributes the stepped-lap joint between the leg and yoke laminations and results in a symmetrical core structure which provides superior electrical characteristics. However, there is an inherent difficulty in constructing a horizontal stepped-lap magnetic core due to the multiple spaced end points of the inner leg laminations which are hidden from the view of the operator during assembly of the core thereby necessitating longer assembly times.
Arrangements for stepping the inner leg laminations in a vertical direction are shown in U.S. Pat. Nos. 3,153,215; and 3,743,991. In this type of magnetic core structure, the equal length inner leg laminations are vertically distributed, parallel to the straight side of the inner leg, by progressively notching one yoke lamination deeper and the other yoke lamination shallower than that of adjoining layers. Alternately, the length of the inner length laminations may be incrementally varied from layer-to-layer to produce a vertical lap joint. In either vertical step lap joint magnetic core structure, the equal length yoke laminations are incrementally shifted in a horizontal direction to form a step lap joint with the leg laminations. U.S. Pat. Nos. 3,670,279 and 3,918,153 both disclose arrangements for constructing a step lap joint with leg and yoke laminations that incrementally change lengths from layer to layer.
Another magnetic core structure of the stacked type is disclosed in U.S. Pat. No. 4,201,966. In that patent the outer legs, inner leg and top and bottom yokes are formed of a plurality of stacked groups of layers of metallic laminations. The length dimensions of the leg and yoke laminations are varied in opposite directions from layer to layer within each group of layers while maintaining the midpoints of the laminations in each leg and yoke portion in alignment. This arrangement offsets the ends of the leg and yoke laminations from layer to layer and provides a step lap joint between adjoining ends of the leg and yoke laminations. The relative locations of the leg and yoke laminations are selected to uniformly divide the voids formed at the inner corners of the magnetic core between the leg and yoke laminations within each group of layers of laminations.
While the foregoing magnetic core structure designs have been successful, they have left something to be desired. It would be desirable to provide a step lap side leg or limb design which is applicable to a three leg single phase core, a four leg single phase core and five leg single or three phase core. It would be desirable to provide a core that is easy to cut and build and where the parts and methods of assembly are less complex than the prior art. It would be desirable to have an arrangement where the inner or center core guides the step lap so that the step lap moves in the horizontal direction and all laminations of the core except the center leg laminations are identical within each group of laminations. Thus, the outer leg and yoke laminations are step independent. It would also be desirable to provide a magnetic core of the stacked type where the probability for consistently low core losses will be high due to the fact that the cores can be built with very tight tolerances.